1. Field of the Invention
The present invention relates generally to semiconductor laser devices, and more particularly to a semiconductor laser device that selectively absorbs a portion of the light radiated from an active layer of the semiconductor laser device.
2. Discussion of the Background
With the recent demand for increased bandwidth for data communications, optical networks and the components essential for their operation are being closely studied. To provide a light source for such optical networks, semiconductor laser devices such as the distributed feedback (DFB) semiconductor laser device, the distributed Bragg reflector (DBR) semiconductor laser device, and the fiber Bragg grating (FBG) semiconductor laser module, or the like have been used. Each of these devices include a wavelength selecting structure capable of selecting a lasing wavelength independent of the optical gain distribution of the active layer and emitting the selected wavelength from the laser device.
For example, FIG. 13 shows a cross section of an exemplary distributed feedback semiconductor laser 1300 (hereinafter, referred to as a DFB laser). As seen in this figure, the DFB laser has an active layer 1301 wherein radiative recombination takes place, and a diffraction grating 1303 for changing the real part and/or the imaginary part of the refractive index (complex refractive index) periodically, so that only the light having a specific wavelength is fed back for wavelength selectivity. The diffraction grating 1303 is comprised of a group of periodically spaced parallel rows of grating material 1305 surrounded by a cladding material 1307 (typically made of InP material) to form a compound semiconductor layer that periodically differs in refractive index from the surroundings. In a DFB laser having such a diffraction grating 1303 in the vicinity of its active layer 1301, the lasing wavelength xcexDFB which is emitted from the DFB laser is determined by the relation:
xe2x80x83xcexDFB=2neffxcex9,
where xcex9 is the period of the diffraction grating as shown in FIG. 13, and neff is the effective refractive index of the waveguide. Thus, the period xcex9 of the diffraction grating and the effective refractive index neff of the waveguide can be adjusted to set the lasing wavelength xcexDFB independent of the peak wavelength of the optical gain of the active layer.
This setting of the lasing wavelength xcexDFB independent of the peak wavelength of the optical gain of the active layer allows for essential detuning of the DFB laser device. Detuning is the process of setting the emitted lasing wavelength of a laser to a different value than the peak wavelength of the optical gain of the active layer to provide more stable laser operation over temperature changes. As is known in the art, a larger detuning value (that is, a large wavelength difference between the emitted lasing wavelength and the peak wavelength of the optical gain of the active layer) can improve high speed modulation or wide temperature laser performance.
In addition to detuning, the lasing wavelength xcexDFB may also be set independent of the peak wavelength of the optical gain of the active layer in order to the obtain different characteristics of the semiconductor laser device. For example, when the lasing wavelength of the DFB laser is set at wavelengths shorter than the peak wavelength of the optical gain distribution, the differential gain increases to improve the DFB laser in high-speed modulation characteristics and the like. Where the lasing wavelength of the DFB laser is set approximately equal to the peak wavelength of the optical gain distribution of the active layer, the threshold current of the laser device decreases at room temperature. Still alternatively, setting xcexDFB at wavelengths longer than the peak wavelength improves operational characteristics of the DFB laser, such as light intensity output and current injection characteristics, at higher temperatures or at a high driving current operation. It is noted, however, that where the lasing wavelength xcexDFB is set shorter or longer than the peak wavelength of the optical gain distribution of the active layer, undesirable lasing may actually occur at the peak wavelength of the optical gain distribution of the active layer. Thus, the peak wavelength is generally suppressed in order to optimize the emitted light at the lasing wavelength.
The conventional DFB laser such as that disclosed in FIG. 13 can be broadly divided into a refractive index coupled type laser and a gain coupled type laser. In the refractive index coupled DFB laser, the compound semiconductor layer constituting the diffraction grating has a bandgap energy considerably higher than the bandgap energy of the active layer and the bandgap energy of the lasing wavelength. Thus, bandgap wavelength (which is a wavelength conversion of the bandgap energy) of the diffraction grating is typically at least 100 nm shorter than the lasing wavelength and is usually within the range of 1200 nm-1300 nm if the xcexDFB is approximately 1550 nm. In the gain coupled DFB laser, the bandgap wavelength of the compound semiconductor layer constituting the diffraction grating is longer than the lasing wavelength and is typically about 1650 nm if the xcexDFB is approximately 1550 nm. FIGS. 14a and 14b show the operational characteristics of an exemplary refractive index coupled laser and gain coupled laser respectively. Each of these figures includes xcexe, xcexg, xcexmax, and xcexInP shown plotted on an abscissa which shows wavelength increasing from left to right in the figures. In this regard, xcexe is the selected lasing wavelength of the DFB laser 1300, xcexmax is the peak wavelength of the optical gain distribution of the active layer 1301, xcexg is the bandgap wavelength of the diffraction grating material 1305, and xcexInP is the bandgap wavelength of the surrounding InP material 1307. As seen in FIGS. 14a and 14b, the bandgap wavelength xcexInP is typically 920 nm and the bandgap wavelength xcexg is closely related to the absorption loss of the diffraction grating which is shown by the broken curves 1403 and 1403xe2x80x2. Moreover, the refractive index of a material increases as the bandgap wavelength of the material increases as shown by the arrows 1405. Thus, as seen in the figures, the refractive index of the diffraction grating having the bandgap wavelength xcexg is generally higher than the refractive index of the surrounding InP layer having the bandgap wavelength xcexInP.
FIG. 14a shows an exemplary refractive index coupled DFB laser wherein the lasing wavelength xcexe which is greater than the bandgap wavelength xcexg of the diffraction grating layer. Specifically, the DFB laser has a lasing wavelength xcexe of 1550 nm, has a bandgap wavelength xcexg of 1250 nm, and satisfies the relationship:
xcexg less than xcexmax  less than xcexe.
Thus, the DFB laser of FIG. 14(a) reflects xcexexe2x88x92xcexg=300 nm. With the refractive index coupled DFB laser, the absorption loss curve 1403 does not cross the lasing wavelength xcexe and therefore absorption loss at xcexe is very small. Accordingly, the DFB laser of FIG. 14a, has the advantage of a low threshold current and favorable optical output-injection current characteristics. However, as also shown in FIG. 14a, in a refractive index coupled DFB laser, the absorption loss curve 1403 also does not cross the peak wavelength of the optical gain distribution of the active layer xcexmax. Therefore, assuming that the absorption coefficient with respect to the lasing wavelength xcexe of the DFB laser is xcexe and the absorption coefficient with respect to the bandgap wavelength of the active layer, or the peak wavelength xcexmax of the optical gain distribution of the active layer, is xcex1max, then xcex1e is approximately equal to xcex1max which is approximately equal to zero. This means that the absorption curve 1403 affects neither xcexmax nor xcexe, and the peak wavelength xcexmax of the optical gain distribution of the active layer is not suppressed with respect to the lasing wavelength xcexe.
More specifically, there is a problem with the refractive index coupled laser in that a side mode suppression ratio (SMSR) of adequate magnitude cannot be secured between the lasing mode at the designed lasing wavelength xcexe of the DFB laser and the mode around the peak wavelength xcexmax of the optical gain distribution of the active layer. In addition, because neither the xcexmax nor the xcexe wavelengths are affected by the absorption curve 1403, wide detuning cannot be accomplished using the refractive index coupled semiconductor laser of FIG. 14a. That is, the absolute value of the detuning amount |xcexexe2x88x92xcexmax| cannot be made greater since an increase in the absolute value of the detuning amount |xcexexe2x88x92xcexmax| would result in a large gain difference between the lasing wavelength xcexe and xcexmax, and lowers the single mode properties and narrows the temperature range operation of the refractive index coupled semiconductor laser.
Finally, with the refractive index coupled DFB laser of FIG. 14a, the difference in the refractive index of the grating material 1305 and the refractive index of the InP buried layer 1307 is relatively small. Therefore, the physical distance between the grating material 1305 and the active layer 1301 of the DFB laser 1300 must be reduced and, as a result, the coupling coefficient varies greatly depending on the thickness of the diffraction grating layer and the duty ratio which is expressed as W/xcex9, where W is the width of one element of the diffraction grating and xcex9 is the pitch of the gratings. This makes it difficult to fabricate refractive index DFB laser devices having the same characteristics resulting in low manufacturing yields for this type of laser.
As seen in FIG. 14b, the gain coupled DFB laser has a lasing wavelength xcexe of which is less than the bandgap wavelength xcexg of the diffraction grating layer. Specifically, the DFB laser of FIG. 14b has a lasing wavelength xcexe of 1550 nm, a bandgap wavelength xcexg of 1650 nm, and satisfies the relationship:
xcexmax less than xcexe less than xcexg.
Thus, this exemplary DFB laser reflects xcexexe2x88x92xcexg=xe2x88x92100 nm. In the gain coupled DFB laser of FIG. 14b, there is a relatively large difference between the refractive index of the grating material 1305 and refractive index of the InP buried layer 1307 which makes it possible to increase the distance between the grating material 1305 and the active layer 1301. As a result, unlike the refractive index coupled DFB laser, the coupling coefficient of the gain coupled laser does not vary with the thickness of the diffraction grating layer and the duty ratio, and same-characteristic DFB lasers can be fabricated with stability thereby allowing higher production yields for this type of laser.
However, as also seen in FIG. 14b, the gain coupled DFB laser has an absorption loss curve 1403xe2x80x2 that crosses the lasing wavelength xcexe and, therefore, absorption loss at the desired lasing wavelength xcexe is large resulting in a high threshold current and unfavorable optical output-injection current characteristics. Moreover, although the absorption loss curve 1403xe2x80x2 also crosses the undesired wavelength of xcexmax, the absorption coefficient xcex1max is approximately equal to the absorption coefficient xcex1e. That is, as with the refractive index coupled DFB laser, the absorption curve 1403xe2x80x2 of the gain coupled DFB laser affects xcexmax and xcexe equally and the peak wavelength xcexmax of the optical gain distribution of the active layer is not suppressed with respect to the lasing wavelength xcexe resulting in a low side mode suppression ratio (SMSR). For example, in the conventional DFB lasers of FIGS. 14a and 14b, the SMSR, though depending on the amount of detuning to the lasing wavelength of the DFB laser, falls within a comparatively small range of 35 and 40 dB. Also like the refractive index coupled DFB laser, since the absorption curve 1403xe2x80x2 affects xcexmax and xcexe equally, wide detuning cannot be accomplished because the wider the spacing between the xcexmax and xcexe wavelengths, the smaller the gain of the desired lasing wavelength xcexe will be with respect to the undesired xcexmax. Thus, whether the xcexe is set shorter or longer than xcexmax, the absolute value of the detuning amount |xcexexe2x88x92xcexmax| of conventional refractive index and gain coupled DFB lasers is limited several tens of nanometers thereby causing unfavorable single mode and temperature range characteristics for these devices.
Accordingly, one object of the present invention is to provide a semiconductor laser device and method which overcomes the above described problems.
According to a first aspect of the invention, there is provided a semiconductor laser device having a semiconductor substrate, an active region formed on the semiconductor substrate and configured to radiate light having a predetermined wavelength range, a wavelength selecting structure configured to select a first portion of the radiated light for emitting from the semiconductor laser device, and an absorption region located in a vicinity of the active region and configured to selectively absorb a second portion of the radiated light, the first portion of the radiated light having a different wavelength than the second portion of the radiated light.
In one embodiment of the first aspect, the first portion of the radiated light is a single mode lasing wavelength xcexe and the second portion of the radiated light is a peak wavelength xcexmax of an optical gain distribution of the active region. In this embodiment, the absorption region is configured to provide operational characteristics satisfying any one of the relationships: 0 less than xcexexe2x88x92xcexabsxe2x89xa6100 nm; 0 less than xcexexe2x88x92xcexabsxe2x89xa670 nm; or xcexexe2x88x92xcexabs=50 nm, where xcexabs is the bandgap wavelength of the absorption region, and xcexe is the single mode lasing wavelength.
In another embodiment of the first aspect, the absorption region of the semiconductor laser is configured to provide operational characteristics satisfying any one of the relationships: xcex1max greater than xcex1e; xcex1maxxe2x88x92xcex1exe2x89xa71 cmxe2x88x921; or xcex1maxxe2x88x92xcex1exe2x89xa75 cmxe2x88x921, in terms of waveguide loss, where xcex1max is an absorption coefficient with respect to the peak wavelength xcexmax of the optical gain distribution of the active region, and xcex1e is an absorption coefficient with respect to the selected lasing wavelength xcexe. In this embodiment, the absorption region may be configured such that the absorption coefficient xcex1e is substantially 0.
In yet another embodiment of the first aspect of the present invention, the active region, wavelength selecting structure, and absorption region are configured to provide operational characteristics satisfying any one of the relationships xcexabs less than xcexmax less than xcexe, or the relationship xcexmax less than xcexabs less than xcexe, where xcexabs is the bandgap wavelength of the absorption region, xcexmax is the peak wavelength of an optical gain distribution of the active region, and xcexe is the single mode lasing wavelength.
In another embodiment of the first aspect, the active region and absorption region are configured to provide operational characteristics such that xcexabsxe2x88x92xcexmax ranges from approximately 10 nm to approximately 20 nm.
In another aspect of the present invention, the wavelength selecting structure of the semiconductor laser device includes an external fiber grating, a distributed Bragg reflector, or an integrated diffraction grating formed on the active region. This aspect of the invention may include each of the operational characteristics of the embodiments described in the first aspect above. In addition, where the wavelength selecting structure includes an internal diffraction grating, the structure includes a group of periodically spaced parallel rows of grating material that extends along a portion of the entire length, or the entire length of the active region on which the diffraction grating is formed. In this embodiment, the grating material includes GaInAsP, and the cladding material includes InP.
In a third aspect of the present invention, the absorption region includes a selective absorption semiconductor layer. This aspect of the invention may include each of the operational characteristics of the embodiments described in the first aspect above. In one embodiment of the third aspect, the selective absorption layer is a quantized layer with a thickness small enough to develop a quantum effect. The selective absorption layer may include InGaAs and have a thickness of approximately 5 nm.
The semiconductor laser device may also have a single mode lasing wavelength xcexe which is greater or less than the peak wavelength xcexmax of the optical gain distribution of the active region and may include an absolute value of a detuning amount of at least 20 nm. The threshold current of the laser device may be no greater than 9 mA.
In another aspect of the present invention, first portion of the radiated light includes multiple oscillation wavelengths.
In yet another aspect of the invention, the absorption region of the semiconductor laser device includes both a diffracation grating and a selective absorption layer mode of a quantized structure. In this aspect, the the diffraction grating selectively absorbs wavelengths shorter than the lasing wavelength xcexe, and the selective absorption layer selectively absorbs wavelengths longer than the lasing wavelength xcexe.
The semiconductor laser device may be used in an optical fiber amplifier such as a raman amplifier, a wavelength division multiplexing system, or a semiconductor laser module.